(a) Field of the Invention
The present invention relates to a system as well as a method for measuring the residual flux present in current transformers and more particularly concerns a method and a system for indicating in a direct manner the value of that residual flux. Moreover, in a preferred embodiment, the present invention incorporates a method and means for cancelling the residual flux through controlled demagnetization of the current transformer core.
(b) Description of Prior Art
It is well known that the presence of periodic components in a transient current causes, in a current transformer, an asymetrical development of flux or induction in the current transformer core while in the transient state. Under such conditions, the transformer core may be driven to saturation and then an error in the ratio of the current transformer may result. The probability of occurrence of such event is higher in the case of closed core transformers, that is with those without an air-gap, for reason of the presence therein of a permanent residual flux. The existence of such residual flux in the core of that type of transformer lowers the dynamic range available for asymetrical induction variations in the transient state. Moreover, in the transient state, during which the induction as raised to a high level after such transient state, a residual flux may be created which substantially reduces the dynamic range of the transformer.
A standard method for measuring the residual flux level in a core, onto which a winding is wound, includes measuring the voltage induced in that winding while the induction varies, and integrating the measured value by means of an electronic integrator. The output signal from the integrator is then approximately equal to the induction variation value from a starting level, which is not known. In that standard method, the starting level which corresponds to the residual flux level must be evaluated.